(image-type): New variable.

(image-type): New variable.
(image-mode): Set default major mode name to "Image[text]".
(image-minor-mode): Change LIGHTER to display image-type in the
mode line.
(image-minor-mode): Set default image-type to "text".
(image-toggle-display): After switching to text mode, set
image-type to "text" and major mode name to "Image[text]".
After switching to image mode, set image-type to actual image
type, and add image type to major mode name.  Let-bind the same
variable names as arguments of `image-type' and `create-image'.
Bind `type' to the result of `image-type' and use it as arg
of `create-image' to not determine the image type twice.